The invention relates to transmitters used in a communications system. More particularly, the invention relates to a phase-locked loop (PLL) of a dual band transmitter used in a wireless phone, such as a dual band cellular phone for mobile communications systems.
Cellular phones are designed to operate within the environment of one of several mobile communications networks. An exemplary environment is a mobile communications network according to a standard known as GSM (Global System for Mobile communications) created by the European Telecommunications Standards Institute (ETSI). In GSM, there is an assigned frequency band around 900 MHz for Standard GSM, and an assigned frequency band around 1800 MHz for DCS1800 (Digital Communications System, DCS). Other environments include systems known as Advanced Mobile Phone System (AMPS) operating in a frequency band around 800 MHz, and as Personal Communications System (PCS) operating in a frequency band around 1900 MHz.
As the number of users of cellular phones increases, some operators have added capacity to their networks by including more than only one frequency band in their networks. In the case of the GSM system, for example, the frequency bands around 900 MHz and 1800 MHz are now used to expand the capacities of certain networks.
Manufacturer of cellular phones, therefore, are developing a new generation of cellular phones which are operable at, for example, two frequency bands. A cellular phone operable at two frequency bands is referred to as a dual band cellular phone. As the dual band cellular phone has to transmit signals at both frequency bands, duplication of functions is required. For example, a second PLL is required in a GSM900/DCS1800 dual band cellular telephone having transmit frequency bands between 890 MHz and 915 MHz for the GSM band, and between 1710 MHz and 1785 MHz for the DCS 1800 band. This would require using two separate voltage-controlled oscillators (VCO), which adds complexity and expense to the over-all system design. Duplication of functions such as VCOs, however, leads to higher manufacturing costs and ultimately to higher prices for the dual band cellular phones.